The present invention relates to a direct access storage device (hereinafter referred to as DASD) shared by a plurality of central processing units.
In Japanese Patent Unexamined Publication Nos. Sho-61-20148 and Sho-61-43356, there is provided a file controller connected independently of a DASD and logically divided into blocks to thereby carry out exclusive control on each block as an access unit.
On the other hand, in a computer system disclosed in Japanese Patent Unexamined Publication No. Sho-61-32151, in which a plurality of central processing units share a pair of duplex DASDs, exclusive control of access to the shared duplex DASDs is carried out by using a hardware reserving function in which the whole of the DASDs is reserved or released by an exclusive control circuit in the DASDs or in a controller of the DASDs, or is carried out by communication of reserve or release requests between the plurality of central processing units.
In a system disclosed in Japanese Patent Unexamined Publication No. Sho-61-62918, as a failure recovery procedure for one of a pair of shared duplex DASDs, the whole of the failed DASD is first blocked and then necessary data sets are reconstructed on a newly provided DASD, so that the operation of the pair of shared duplex DASDs is continued.
Further, Japanese Patent Unexamined Publication No. Sho-61-67153 discloses a failure recovery method in which, in a case where a failure occurs in a DASD, a reserved area preliminarily secured is assigned as an alternative area to avoid a portion where the failure occurs.
In the aforementioned conventional technology, in case of an occurrence of a failure in the DASDs, blocking control is carried out by DASD. In short, in the case where a partial failure occurs in a DASD, a reserved area is assigned as an alternative area for the failure portion so that the central processing unit can operate with no sense of the partial failure. Accordingly, not only in the case where access onto the whole DASD becomes impossible, but in the case where partial failure occurs frequently to make the alternative area insufficient, the blocking procedure by the DASD is carried out.
For example, in a magnetic disk device which is a kind of DASD, in most cases a "track" is used as a physical failure control unit. In other words, when a disk controller which has accepted an input/output request from a central processing unit detects an access failure portion in which reading/writing is impossible, the disk controller executes an alternative track assignment to assign a track containing the failure portion to a reserved track. However, when a partial failure occurs so frequently that all alternative tracks are drained, the whole disk device must be blocked even in the case where an access failure of one track occurs in the disk device.
In the case that one DASD in a pair of duplex DASDs is blocked as described above, and a stationary access failure occurs on the other DASD, it becomes impossible to obtain access on all the duplex data sets on the duplex DASDs.
In other words, even when a partial failure occurs under the condition that alternative areas such as alternative tracks are insufficient, duplex access on the whole of the pair of duplex DASDs is made impossible. There arises a problem in that reliability on the DASDs is lowered remarkably compared with the case where duplex access is being executed.
In the case where duplex access can be made, for example, at the time of access to different data sets, at the time of read only access, etc., a plurality of CPUs can obtain access to individual DASDs in parallel. However, when duplex access is made impossible by the blockade for one DASD, access from the plurality of CPUs is concentrated onto the other DASD to spoil the property of parallel processing of dispersing access to the two DASDs. Therefore, there arises a problem in that processing performance of the computer system is lowered.
In the aforementioned conventional technology, in order to construct a computer system using a pair of shared duplex DASDs, it is necessary that communication paths be between the CPUs for exclusive control or communication paths between the CPUs and a file, and a controller for executing exclusive control are provided so that the CPUs execute reserve or release of the data sets according to a specific communication procedure. In other words, in the aforementioned conventional technology, the same communication path as used in a standard input/output device is used both in the case of communication paths between the CPUs and in the case of communication paths between the CPUs and a file controller. Accordingly, a series of procedures such as command generation and data generation for the input/out channels, the starting of the input/output channels, the input/output interrupt waiting, the input/output interruption, and the like, is required whenever communication is made. In a small-scaled computer system, processing overhead caused by the communication procedure cannot be neglected.
Further, in the conventional technology in which a file controller is used to control access by a data set, there is a risk that when a plurality of CPUs are attempting access to the file controller simultaneously, one of the CPUs frequently obtains access to the file controller so that the access of the other CPUs to the file controller is made difficult.